drm/amd/display: Improve spelling, grammar, and formatting of amdgpu_dm.c comments

[Why]
Good spelling and grammar makes comments
more pleasant and clearer.

Linux has coding standards for comments
that we should try to follow.

[How]
Fix obvious spelling and grammar issues

Ensure all comments use '/*' and '*/' and multi-line comments
follow linux convention

Remove line-of-stars comments that do not separate sections
of code and comments referring to lines of code that have
since been removed
Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>